Being in the field of electronics (not in computers though) I know something about a branded and an assembled commuter. There is a big difference in quality as well prices too but in the long run you get benefit if you go for the branded one as every part of it stands its own guarantee as the whole unit where as you will have to depend on guarantee from different sellers (if any) if you go for assembling your own.
Personally, I think it would be penny wise and pound foolish to buy assembled computers over branded ones. The truth is that the assembled one would be a lot cheaper compared to branded ones like HP, and Dell but the catch in there is that branded computers would always outlast the assembled ones by far margin. Also, there is always a compatibility issues with the parts of assembled system. This is the major problem such computers have, and should it develop any new fault over one part, it's going to be difficult in replacing it thereby making the computer a waste of money. Branded computers are my choice, because it's a lot more convenient to use on the long run.
I would and I do assemble them myself, it's far cheaper that way and I get to pick all the components I want at the best prices. Since I know quite a lot about PCs and hardware, compatibility issues are very rare and usually boil down to defective components. I honestly think quality of the branded computers is overstated/exaggerated anyway, but buying them is still probably the best idea for people not knowledgeable in this stuff.
I have been building computers for the last 18+ years, but my last 2 PC were name brand PC's because you can't buy the parts and build the computer for what I paid for the computer. About 2 years ago I bought a Dell XPS 8900 and I had nothing but problems with it. It would freeze all the time and I had Dell do diagnostics on it 10 times and each time they said that there was nothing wrong with it. And I sold it for a loss just to get rid of it. And the PC I have now is a 3-month-old ASUS and the best PC that I have ever owned.
You are right, having knowledge about computer hardwares and it's compatibility with several other ones needed to assemble a full system is the major thing to consider before thinking of having a assembler your own system by yourself. Reading experience from a book or watching a YouTube video isn't going to be enough for you to do this effectively. In order to avoid wasting your time and money by doing it wrong, it's definitely better to go for a new branded already assembled system by a known company.
I honestly prefer a branded computer over assembling one myself.. I feel as though it is an easier and less costly method to get things done rather than simply assembling it yourself. There is the chance that in the process of building you may break something or something simply doesn't add up when trying to build it.
